test: add browser acceptance flow and real playground screenshot - #13

What changed

• adds Playwright browser acceptance coverage for the full Docker Compose playground;
• starts the Java backend and React/Nginx frontend in CI;
• loads the built-in knowledge-base document through the real browser UI;
• submits a grounded question and verifies that the answer contains the cited deployment instruction;
• verifies the evidence panel exposes the same source document, chunk metadata, similarity label, and original passage;
• captures a full-page screenshot from the accepted browser state;
• publishes that accepted screenshot into both English and Chinese README files;
• uploads the screenshot, HTML report, traces, videos, and failure artifacts for inspection;
• moves CI browser tooling to Node.js 22 and pins Playwright Test 1.61.1.

Acceptance path

Docker Compose
→ browser opens Java-RAG Playground
→ load sample document
→ ask how to start the visual playground
→ answer contains docker compose up --build [1]
→ right panel shows matching source and evidence
→ save docs/assets/playground.png
→ render the same accepted screenshot in both READMEs

Why

The visual playground previously had compile-time checks but no real browser validation. This PR turns the README screenshot into reproducible evidence generated from the same end-to-end path users run locally, instead of a hand-built mockup.

Validation

CI performs Java tests, the CLI demo, the React production build, Compose validation, container startup, HTTP readiness checks, Chromium E2E, screenshot publication, artifact upload, and deterministic teardown.

The accepted browser run completed successfully before the screenshot and README showcase were committed back to the PR branch.